1. Field of the Invention
The present invention relates to an image processing method of backlight illumination control, and more particularly, to adjust the displaying illumination of a backlight by remapping the gray-level values of the pixels in an image.
2. Description of Related Art
With great advance in the techniques of electro-optical and semiconductor devices, flat panel displays, such as liquid crystal displays (LCD), have enjoyed burgeoning development and flourished in recent year. Due to the numerous advantages of the LCD, such as low power consumption, free of radiation, and high space utilization, the LCD has become the main stream in the market. An LCD includes a liquid display panel and a backlight module. The liquid display panel has no capacity of emitting light by itself so that the backlight module is arranged below the liquid display panel to provide the surface light source for the liquid crystal display panel so as to perform the display function.
When an image is displayed through the LCD, a driving voltage is applied to the liquid crystal for controlling a rotation angle of the liquid crystal and thereby controlling a light transmission of the liquid crystal, which the driving voltage is related to a gray-level value of the image. Nevertheless, the liquid crystal display panel has non-linear light transmission with respect to the driving voltage so that a gamma correction apparatus is needed to adjust the driving voltage.
FIG. 1 is a curve diagram of light output over the driving voltage with no gamma correction. Referring to FIG. 1, the light output and the driving voltage are normalized to maximum 1 and the lower driving voltages (or the darker gray-level values of the image) are displayed too dark due to the lower light transmission. There are two categories of gamma correction methods. One is analog gamma correction method and the other is digital gamma correction method.
The analog gamma correction method utilizes a plurality of resistances in series connection to adjust the driving voltage directly according to a fixed gamma curve, such as γ=2.2, which the driving voltage is converted from the gray-level value of the image. The digital gamma correction adjusts the gray-level value of the image according to the fixed gamma curve so that the adjusted gray-level value of the image can be converted to a proper driving voltage for controlling the light transmission of the liquid crystal.
Besides, if the backlight illumination decreases, such as power saving, the whole luminance of the image displayed would decrease on the premise of no gamma correction. The designer utilizes the higher gamma curve to adjust the gray-level value of the image so as to enhance the backlight illumination for better visual quality. FIG. 2 is a curve diagram of the backlight illumination over the gray-level value of the image. Referring to FIG. 2, the curve 201 and the curve 202 respectively represents the backlight illumination over the gray-level value of the image when the duty cycle of providing the backlight is 100% and 80%. Apparently, in the same coordinate of the gray-level value, the curve 202 has lower backlight illumination than the curve 201.
The curve 203 represents the adjusted backlight illumination over the gray-level value by utilizing the gamma curve, such as γ=2.0, when the duty cycle of providing the backlight is 80%. This adjustment of the gamma correction is restricted on that the adjusted backlight illumination of the curve 203 can't match the backlight illumination of the curve 201 when the gray-level value gets higher. Therefore, how to solve this problem becomes an important issue to be researched and discussed.